Stimulation of an immune response is dependent upon the presence of antigens recognized as foreign by the host immune system. Bacterial antigens such as Salmonella enterica and Mycobacterium bovis BCG remain in the phagosome and stimulate CD4+ T-cells via antigen presentation through major histocompatibility class II molecules. In contrast, bacterial antigens such as Listeria monocytogenes exit the phagosome into the cytoplasm. The phagolysosomal escape of L. monocytogenes is a unique mechanism, which facilitates major histocompatibility class I antigen presentation of Listerial antigens. This escape is dependent upon the pore-forming sulfhydryl-activated cytolysin, listeriolysin O (LLO).
There exists a long-felt need to develop compositions and methods to enhance the immunogenicity of antigens, especially antigens useful in the prevention and treatment of tumors and intracellular pathogens.